A known headbox of the type described above has engagement members in the form of oblong engagement bodies or engagement dowels arranged in a row extending in the cross machine direction at the upstream end portion of the vane. The engagement dowels have portions protruding from the vane to cooperate with the support walls of the connection bar. The vane is influenced during operation both by a shearing force in the machine direction caused by stock flowing along the vane, as well as a retaining force directed against the machine direction exerted on the engagement dowels by the support wall situated downstream. It is intended that the retaining force during operation be distributed uniformly among the engagement dowels. In practice, however, the retaining force may be distributed non-uniformly among the engagement dowels so that the shearing force on the vane gives rise to local compressive stresses in the cross machine direction in the downstream end portion of the vane. Where compressive stresses arise the vane can buckle, making its downstream end portion uneven, which is not desirable, particularly at a separating vane that separates two layers of stock, since good layering of stock is dependent on a flat separating vane. If the separating vane is not flat, streaks having a grammage different from the rest of the paper web may appear, for instance.
The above-mentioned compressive stresses may arise as a result of variations in the placing of the engagement dowels within a predetermined tolerance interval. The placing of the engagement dowels within the tolerance interval may, for instance, deviate from an ideal placing in such a way that certain engagement dowels are downstream of the other engagement dowels, in which case the retaining force will be distributed in an uncontrolled manner between the engagement dowels, with the risk of compressive stresses appearing in the downstream end portion of the vane, resulting in buckling.
Compressive stresses may also appear in a vane consisting of a plastic material, e.g., glass fiber-reinforced epoxy resin, and having reduced thickness in the machine direction so that the downstream end portion of the vane is relatively thin in relation to the upstream end portion. A vane of plastic material absorbs water from the surroundings both during storage prior to mounting, and also after mounting in the headbox when the vane absorbs liquid from the stocks. As a result of the differences in thickness, the thinner downstream end portion of the vane will become saturated earlier than the thicker upstream end portion of the vane. As the downstream end portion becomes saturated in the direction away from the downstream edge, the downstream end portion lengthens in the cross machine direction, whereas the thicker, unsaturated upstream end portion of the vane retains its dimensions. The extension of the vane at the downstream edge results in the downstream edge of the vane endeavouring to assume a convex form and its upstream edge a concave form. When such a partially saturated vane is influenced during operation by the shearing force from the stocks, the retaining force will be distributed non-uniformly between the engagement dowels so that the downstream end portion of the vane becomes buckled.